The present invention relates to Raman effect optical amplifiers used in particular in telecommunications optical systems.
Raman amplifiers have properties that are advantageous for wavelength division multiplex (WDM) optical transmission systems because they present a wide passband, low noise, and low sensitivity to variations in signal input power.
Nevertheless, in order to obtain effective amplification, it is necessary to use a high power light pump and an optical fiber of considerable length, thereby making Raman amplifiers expensive.
Raman amplification conventionally uses non-resonant stimulated Raman diffusion. In amplification of that type, and as shown by the FIG. 1 diagram of energy W, a pump wave photon at a wavelength λp loses its energy to create another photon of smaller energy and thus longer wavelength λd at a lower frequency (inelastic diffusion). The remainder of the energy of the incident photon is absorbed by the propagation medium (optical fiber) in the form of molecular vibration (phonon) represented by the vertical arrow between a solid first horizontal line 1 symbolizing a reference energy level corresponding to a fundamental vibration state and a thick solid second horizontal line 2 symbolizing a band of energy levels corresponding to excited vibration states. In order to increase amplification effectiveness, it is known to reduce the effective section area of the optical fiber so as to increase confinement, and/or to dope the core of the fiber strongly with a material having a high Raman coefficient, such as germanium in the form of an oxide forming part of the matrix constituting the core. Although core doping increases the effectiveness of the amplification, it nevertheless has the corollary of increasing losses.
To explain that conventional type of Raman amplification, the above description relates to a virtual electron energy level represented in the figure by a dashed horizontal line 3 for showing the transfer of energy from the pump wave at λp to the signal for amplification at λd. This level is “virtual” since it does not correspond to any stable energy state of atoms constituting the core, including any possible dopants. This description of the Raman process in terms of a “virtual” level is thus justified by the fact that the interaction between the photons and the molecules, and the re-emissions of photons take place quasi-instantaneously.